1. Field of the Invention
The present invention relates to scanning laser radar systems for mapping the three-dimensional features of objects in a scene.
2. Description of the Related Art
Effective automation techniques for industry and business often require remote sensing to ascertain information about the particular process being monitored. A variety of sensors have come into existence for such remote sensing purposes, including sensors for measuring temperature, pressure, color, hardness, volume and distance. These devices have solved their respective sensing requirement to varying degrees of success.
Many sensing uses require quantification of the spatial relationship between objects in a scene, which generally involves describing the size, shape and geometry of objects in the scene as well as the spatial relationships between individual objects in the scene. Such spatial data is useful for manufacturing processes, metrology processes, materials handling, automated warehousing, quality control, security, robotic vision, and human vision substitutes, for example. One particularly promising sensor is a scanning laser radar, which uses visible or infrared lasers to obtain information about the scene of interest. Due to the small wavelengths of such lasers, the potential exists to perform very accurate measurements. Furthermore, the continuing development of laser diodes provides a cost effective source of laser radiation and the ability for high modulation rates. Broadly speaking, the laser scanner generates an intensity-modulated laser probe beam that is reflected off the object whose range is to be determined. A reference signal is generated in a waveform generator and this signal then drives a laser diode to produce an intensity-modulated waveform.
The amplitude-modulated laser beam is directed to a scanning system that includes at least a horizontal scanning mirror driven by a constant-speed motor. Many scanning systems also use a vertical scanning mirror orthogonally positioned with respect to the horizontal scanning mirror. The combination of the horizontal scanning mirror and the vertical scanning mirror allow the laser beam to scan an entire area with the intensity-modulated laser beam. Upon reflection from the object, the intensity modulation continues in the reflected beam and as a result, the reflected beam has the same type of intensity modulation as the probe beam, but with a different phase due to the additional distance it travels. When the reflected beam returns, the light energy is converted to electrical energy by a photodetector, and the phase of the intensity-modulated reflected beam is compared with the phase of the reference signal and a distance measurement can be obtained.
Generally, as the amplitude modulation becomes higher in frequency, accuracy increases. However, high frequency electrical signals, for example a 100 MHz signal, can be substantially distorted during subsequent electrical processing and particularly the zero crossings can be shifted, which can substantially degrade accuracy of the subsequent phase measurement. In previous systems, mixers have been used to down-convert the original frequency of the amplitude-modulation to a lower frequency before processing, thereby allowing more accurate, lower frequency processing. However, there is still room for improvement in signal processing.
Other range sensing technologies include television sensors, imaging sensors utilizing CCD arrays, acoustic ranging sensors, and radar.
Television sensors are available which operate in the visible or near infra-red parts of the electromagnetic spectrum. Although an individual sensor provides two-dimensional (2-D) imagery, techniques have been developed for obtaining 3-D scene descriptions through use of techniques such as binocular vision and structured light. These approaches have never been widely accepted because of the computational complexity and time involved in extracting the desired scene data. In addition, controlling factors such as scene lighting balance, lighting intensity, etc. have proven to be very difficult.
Imaging sensors utilizing CCD arrays exist for obtaining images of a scene. Such CCD arrays can operate in the visible, near infra-red or the far infra-red (10-14 micrometer wavelengths). CCD techniques are analogous to the TV sensor approach described above with the exception that the far infra-red band provides a solution for operating any time of day by exploiting specific wavelengths of natural emissions from scene objects at temperatures around 300.degree. K. However, extraction of 3-D data has the same limitations as TV.
Acoustic ranging sensors also exist, similar to Kodak camera acoustic ranging sensors. Techniques using such acoustic ranging sensors are unsuitable for imaging uses because of the inherent lack of precision of range measurement and the wide beam angle.
Radar systems operate in a variety of different wavelengths, from many meters to millimeters. Radar technology has been developed to a high degree for military and aviation applications; however, wide beam angles and lack of required ranging accuracy prevent their successful application in industry and manufacturing.